Blender 2.8 / Windows 10 / Python - import Module fails, pip install Module fails, pip install Wheel fails
I'm not sure if the reinstall and admin rights are necessary for all cases but the following solved my problem:


Uninstall Blender and reinstall as Admin
In Blender Python shell run 

import sys
sys.exec_prefix

Open Powershell as Admin
Go to directory listed by Blender Python shell
Ensure pip

.\bin\python.exe -m ensurepip

Install modules

.\Scripts\pip3.exe install vtk



Thanks to @rgj for troubleshooting and this blog for Blender/pip instructions